1. Field of the Invention
The present invention relates to intermediate-frequency circuits for television (TV) tuners.
2. Description of the Related Art
FIG. 4 shows a known intermediate-frequency circuit. First, a selected TV signal (RF) and a local oscillator signal (LO) are fed to a mixer 31. The resulting intermediate-frequency signal from the mixer 31 is then fed through an intermediate-frequency tuning circuit 32 to an intermediate-frequency amplifier 33. The intermediate-frequency tuning circuit 32 is a parallel-connected tuned circuit in which series-connected inductance elements 32a and 32b and a capacitive element 32c are connected in parallel. A voltage B for feeding to the mixer 31 is applied to a junction between the two inductance elements 32a and 32b. The mixer 31 and the intermediate-frequency amplifier 33 form a balanced circuit. The intermediate-frequency tuning circuit 32 is connected to two balanced lines connecting the output terminal and the input terminal of the mixer 31 and the intermediate-frequency amplifier 33, respectively.
An intermediate-frequency signal from the intermediate-frequency amplifier 33 is fed through a surface-acoustic-wave (SAW) filter 34 to a video detector 35, while a local oscillator signal for synchronous detection is fed from a voltage-controlled oscillator (not shown) to the video detector 35. A video signal is then transmitted from the video detector 35.
In the National Television System Committee (NTSC) mode, over the frequency range from 0 (direct current) to about 4 MHz, a video signal is required to exhibit a flat characteristic as well as constant group delay time, which is substantially determined by a video detector and a SAW filter. Specifically, the group delay time in the frequency band ranging from 60 Hz to 1 MHz may be shortened by about 50 to 100 ns as compared to that in the frequency band ranging from 2 to 3 MHz. In this case, the group delay time in the lower band (60 Hz to 1 MHz), which leads to overshoot and undershoot of the video signal, cannot be easily corrected (see FIG. 5).